Method for controlling a communications channel shared by several stations

ABSTRACT

A communication system able to provide multiple path communication between a plurality of stations operating on a single channel. The stations are synchronized and a cyclically repeating series of time slots is defined. One time slot in each cycle is reserved for the transmission of synchronization information by a station designated the master station for reception by the other stations, designated slave stations, and maintaining synchronization therein. Another time slot is reserved for any slave station to transmit a message that it requires to communicate to another station, such indication preferably being by transmitting its own pre-assigned address code. The remaining time slots are used for transmitting address information and data. The slave stations operate in a low power condition except during one of the other time slots when they may receive their own address, or except when they need to transmit data.

[0001] The present invention relates to the field of telecommunications,and in particular to that of professional mobile radiocommunications.

[0002] Most professional mobile radiocommunications systems use theprinciple of “trunking”, that is to say the network possesses aninfrastructure which manages the use of the radio resources, throughoutthe duration of the communications. The network allocates the resourcesto the communications and controls the access of the terminals to theair interface.

[0003] Systems without “trunking” also exist, in which mobile terminalscan communicate in direct mode or in relayed mode, that is to say by wayof repeaters. In general, the voice communications are of thewalkie-talkie type, with channel access on an alternate turns basis. Theterminal listens to the frequency of the channel so as to determinewhether it has the right to transmit when the user actuates a PTT(“push-to-talk”) button with which the terminal is provided. The rightto transmit is granted if the channel is free or busy with acommunication over which the terminal has priority.

[0004] In this latter type of system, the call with off-hook services(FOACSU: “full off-air call set-up”) of telephone type are notsupported. Specifically, in the absence of a base station or repeatermanaging the connection and access to the radio channel, a communicationcan be interrupted by an external user, during a silence on the channeldue to the halting of voice transmission.

[0005] An aim of the present invention is to propose a way of supportingthe services of call with off-hook type in radiocommunications systemswithout “trunking”.

[0006] The invention thus proposes a method for controlling a channelshared by several transmitting/receiving stations, comprising at leastone phase of use of the channel by a station participating in acommunication, in the course of which said station transmits trafficinformation on the channel, followed immediately by a phase of retentionof the channel by said station, in the course of which said stationtransmits signaling information authorizing each other stationparticipating in the communication to transmit in turn trafficinformation on the channel. This phase of retention of the channel bysaid station continues, so long as the communication is in progress,until another phase of use of the channel by a station participating inthe communication.

[0007] A transmitting/receiving station is master of the channel when ittransmits traffic signals (typically speech). In the case where thestation is a mobile terminal, when its owner stops talking, the stationnevertheless continues to transmit the signaling information on theradio resource employed by the communication. This allows the slavestations participating in the communication to take their turn to speak,and prevents the other stations, not involved in the communication, fromtrespassing by believing the channel to be free.

[0008] The signaling information, transmitted at least between thespeech alternations, can comprise data identifying a set of stationspermitted to participate in the communication, and/or data indicating alevel of priority of the communication, and/or a reference foridentifying the communication. On receiving this information, atransmitting/receiving station different from the master station candetermine whether it has the right to transmit on the radio resource,that is to say whether it is participating in the communication (ofpoint-to-point or group call type) or whether it has a right of channeloverride. The station which starts transmitting then becomes the newmaster station, and the previous master station becomes a slave station(unless it is this station which has resumed speaking).

[0009] Another aspect of the invention pertains to a station fortransmitting/receiving on a shared channel, implementing the methodhereinabove and comprising:

[0010] means of use of the channel in master mode in the course of acommunication, which are designed to transmit traffic information on thechannel;

[0011] means of retention of the channel in master mode in the course ofa communication, which are designed to alternately transmit signalinginformation on the channel and to listen to the channel, said signalinginformation authorizing each other station participating in thecommunication to transmit in turn traffic information on the channel;

[0012] means of reception in slave mode for receiving traffic andsignaling information transmitted in master mode by another stationparticipating in the communication; and

[0013] means of control for activating the means of use of the channelin master mode in response to a request for use when signalinginformation authorizing the transmission of traffic information has beenreceived by the means of reception in slave mode, so as to activate themeans of retention of the channel in master mode in response to adeactivation of the means of use of the channel in master mode, and toactivate the means of reception in slave mode in response to thereception of signals originating from another station participating inthe communication by the means of retention of the channel in mastermode listening to the channel.

[0014] Other features and advantages of the present invention willbecome apparent in the following description of nonlimiting exemplaryembodiments, with reference to the appended drawings, in which:

[0015]FIG. 1 is a diagram illustrating a radiocommunication system towhich the invention may be applied;

[0016]FIGS. 2 and 3 are charts showing exemplary frame structuresuseable in a method according to the invention; and

[0017]FIG. 4 is a schematic diagram of a transmitting/receiving stationaccording to the invention.

[0018]FIG. 1 diagrammatically shows a professional radiocommunicationsystem 1 supplying telephony services to owners of mobile terminals (MS)2-5. The system comprises radio transmitting/receiving stationscomprising the mobile terminals 2-5, possibly repeaters 6 serving toboost the radio coverage of the system, and possibly interfaces 7serving as gateways with one or more switches 8 of external networks(for example switched network or cellular network) so as to allowtelephone calls between the owners of the mobile terminals 2-5 andexternal subscribers.

[0019] The fixed stations 6, 7 of the system 1 do not necessarily offera network service for supervising the radio communications in progress.The control of access to the channel is dealt with by the variousstations involved in the communication, one of them being a masterstation and the other or others slave stations. In the course of acommunication, the allocating of the role of master station is performedaccording to the speaking turns taken by the parties.

[0020] In the illustration of FIG. 1, the mobile terminal 2 is themaster station at the instant considered, in a communication alsoinvolving the mobile terminals 3 and 4 in direct mode.

[0021] By way of example, each mobile terminal 2-5 comprises a PTT(“press-to-talk”) button which the user actuates in order to take histurn to speak. In response to this actuation, the terminal assesseswhether it can access the channel supporting the communication. If itcannot, the speaking turn is denied. In the case of a gateway 7, therole of the actuation of the PTT button is played by the detection of aspeech signal to be relayed originating from the switch 8.

[0022] By way of example, in the particular case of a frequency divisionmultiple access (FDMA) system, each physical channel serving to supporta communication consists of a frequency available to the system. In thecase where a repeater 6 is involved, the latter can apply atransposition of this frequency so as to avoid the problems ofinterference between the radio signals which it transmits and thosewhich it receives.

[0023]FIG. 2 shows an exemplary frame structure adopted in a phase ofuse of the channel by a master station currently transmitting trafficinformation in the course of a voice communication in an FDMA radiosystem according to FIG. 1. In this example, superframes of 1440 msfollow one another on the relevant frequency. This superframe is dividedinto four parts of like duration, each comprising an interval of 320 msfor the transmission of traffic information (voice signals) over atraffic channel TCH, and an interval of 40 ms for the transmission ofsignaling information.

[0024] One interval of 40 ms out of two is devoted to a logical channelfor signaling CS/FS from the master station to the slave stationsparticipating in the communication. This logical channel carries asynchronization pattern, known a priori to the stations of the system,and serving to synchronize them with the frame structure generated bythe master station. The CS (“channel status”) information advises thestations other than the master station as to the channel busy state, andcomprises in particular:

[0025] the identity or address in the system of the transmitting station(master);

[0026] the address of the destination station (case of a point-to-pointcommunication), or in the case of a group call the addresses of thedestination stations, which may be concatenated into a group address;

[0027] a reference number for the communication in progress;

[0028] an identification of the system 1;

[0029] the level of priority of the communication in progress;

[0030] the state of the channel.

[0031] The FS (“forward signaling”) information supplies signalingprovided for in the system from the master station to the other stationor stations engaged in the communication in progress.

[0032] In the 40 ms interval devoted to the CS/FS logical channel, thetransmission may be performed in the following order:

[0033] the FS information, intended for stations already synchronized;

[0034] the synchronization pattern allowing these stations to track thesynchronization and allowing the other stations to acquire it;

[0035] the CS information which the synchronized stations can decode todetermine whether a communication relevant to them is underway and towhat extent they can transmit on the channel.

[0036] The other time intervals of 40 ms of the superframe according toFIG. 2 serve for the reception of information by the master station.This information pertains to two logical channels:

[0037] RS (“reverse signaling”) supplying signaling provided for in thesystem to the master station from the other station or stations engagedin the communication in progress, and allowing for example the slavestations to respond to interrogations formulated by the master stationin the FS signaling or to signal their entry or their withdrawal from agroup communication;

[0038] TI (“terminal interrupt”) allowing terminals of the system, whichmay be external to the communication in progress, to request access tothe channel for emergency calls. The logical channel TI is also listenedto by the slave stations which are not currently transmitting on the RSlogical channel.

[0039]FIG. 3 shows an exemplary frame structure useable in a call setupphase in the radio system of FIG. 1. The same structure can be adoptedwhen a voice communication is in progress in a phase of retention of thechannel by the master station, that is to say while the master stationhas stopped transmitting traffic information and before another stationstarts transmitting (thus becoming the new master station using thechannel). In this example, frames of 160 ms follow one another on therelevant frequency, and are each divided into four intervals of 40 ms.The first interval contains information of the CS type such as describedpreviously, accompanied by the synchronization pattern. The thirdinterval is also provided so as to be able to contain information of theCS type. The second and fourth intervals belong to the aforesaid TIlogical channel.

[0040] In the setup phase, the station which has the call initiative(master) generates the frame structure according to FIG. 3 and sends, inthe first interval of 40 ms, CS information identifying the calledstation or stations, the coding of the state of the channelcorresponding to a call setup. Each called station (slave) detecting thesynchronization pattern then has available the third interval of 40 msof the frame to respond to the calling station. If a single exchange ofCS type information is not sufficient to set up the communication inaccordance with the call protocol employed, a dialog can take placebetween the relevant stations, the master station using the firstintervals of the frames and the slave stations using the third intervalsto transmit signaling information, the state of the channel indicatedrepresenting the state of advance of the call setup procedure (ringing,off-hook, etc.). The stations can also use FS/RS type signaling. Oncethe setup phase is complete (the called party has gone off-hook), anyone of the stations participating in the communication takes its turn tospeak and becomes master of the channel by generating the framestructure according to FIG. 2.

[0041] In the phase of retention of the channel by a master stationhaving stopped transmitting on the TCH logical channel, this masterstation transmits the CS information in the first interval of 40 ms ofthe frame according to FIG. 3, with a coding of the state of the channelindicating to the slave stations involved in the communication that theyare authorized to seize the channel. By detecting this CS information,the slave stations synchronize themselves with the frame structure ofFIG. 3. If one of them seizes the channel (actuation of the PTT button),it transmits a first block of CS information in the third interval of 40ms of the frame and it becomes master by adopting the frame structureaccording to FIG. 2. The master station retaining the channel listensduring the third interval of 40 ms of the frame. When it detects thereinthe CS information transmitted by the other station currently seizingthe channel, it abandons the master station status henceforth held bythis other station. If the turn to speak is seized again, first, by theowner of the master station retaining the channel, this station remainsmaster of the channel.

[0042] The first block of CS information transmitted by the stationauthorized to seize the channel could also be located in the second orfourth intervals of 40 ms since the master station is listening duringthese intervals.

[0043] The stations external to the communication detect thesynchronization pattern accompanying the CS information in the firstinterval of 40 ms of the frame according to FIG. 3, thus noting that thechannel is busy. They can optionally decode the CS information whichfollows in the first interval, thereby allowing them to verify whetherthey can enter into the communication (station targeted by a group callwhich was not previously participating in the communication; this is the“late entry” function also supported by the CS information in the phaseof use of the channel by the master station) or whether they have aright of channel override in view of the priority level indicated. Ifnot, they are prevented from accessing the channel, other than totransmit TI information in the case where they must put through anemergency call.

[0044] A basic diagram of a transmitting/receiving station isrepresented in FIG. 4. The antenna 10 of the station is linked to itsradio stage 11 corresponding to the analog part of the radio station.

[0045] For the reception part, the baseband signal delivered by theradio stage 11 is supplied to a synchronization unit 12 and to ademodulator 13. The unit 12 searches for the synchronization patterns ofthe signal received, in particular the synchronization patternsaccompanying the CS information. The demodulator 13, synchronized by theunit 12, estimates the symbols sent on the basis of the baseband signal,and supplies these estimated symbols to the processing circuits 14 ofthe station.

[0046] A unit 15 controls the status, master or slave, of thetransmitting/receiving station, by delivering a M/S bit to a framegenerator 16 which delivers sequencing signals complying with the framestructure according to FIGS. 2 or 3. The unit 15 also controls thegenerator 16 with a bit W to indicate thereto that the frames must beaccording to FIG. 2 (W=1, phase of use of the channel) or according toFIG. 3 (W=0, call setup or channel retention phase). When the station isa master terminal (for example M/S=1), the frame structure is generatedwith reference to a local clock. When the station is slave (M/S=0), theframe structure is generated in accordance with the synchronizationinformation acquired by the unit 12 from the signal received.

[0047] The frame generator 16 controls the radio stage 11, the modulator19 and the demodulator 13 so as to place the station in terms oftransmission or reception in the appropriate time intervals according tothe frame structure in force.

[0048] The frame generator 16 also undertakes the sequencing of thecircuits 14 for processing the signal received, and that of the circuits17 for processing the signals to be transmitted. FIG. 4 illustrates ineach block 14, 17 the circuits 20-23, 24-27 for processing the logicalchannels mentioned with reference to FIGS. 2 and 3.

[0049] When the station is master, the bit M/S=1 delivered by the unit15 operates the switches 18 into the position M represented in FIG. 4.In the channel use phase, the signal transmitted, supplied to the radiostage 11 by the modulator 19, originates from the circuits 20, 21 forprocessing the logical traffic channel TCH and the logical signalingchannel CS/FS, which are clocked by the frame generator 16. In theremaining intervals of 40 ms of the frame according to FIG. 2, thecircuits 24, 25 of the master station listen to the RS and TI logicalchannels so as to receive the signals which may be transmitted by theother stations. In the call setup or channel retention phase, the signaltransmitted in the first intervals of 40 ms of the frames originatessolely from the circuit 21 for processing the CS/FS logical channel,clocked by the frame generator 16. In the other three intervals of 40 msof the frame according to FIG. 3, the circuits 24, 25 of the masterstation listen to the CS/RS and TI logical channels so as to receive thesignals which may be transmitted by the other stations.

[0050] When the station is slave, the switches 18 are operated into theposition S of FIG. 4. In the phase of use of the channel by anotherstation, the frame generator 16 activates at the appropriate instantsthe circuit 22 for processing the RS logical channel for thetransmission part and the circuits 25-27 for processing the TI, CS/FSand TCH logical channels for the reception part, in accordance with theframe structure of FIG. 2 generated by the master station. In the phaseof call setup or of retention of the channel by another station, theframe generator 16 activates at the appropriate instants the circuit 22for processing the CS/RS logical channel for the transmission part andthe circuits 25, 26 for processing the TI and CS/FS logical channels forthe reception part, in accordance with the frame structure of FIG. 3generated by the master station.

[0051] The frame generator 16 also operates the circuit 23 fortransmission processing on the TI logical channel in the case of theterminals external to the communication in progress (switches 18 in theposition S) so as to allow them to possibly place an emergency call.Otherwise, the circuit 25 for reception processing on the TI logicalchannel is activated in the corresponding intervals so as to allow thestation to pick up a possible emergency call.

[0052] Terminals according to FIG. 4 can communicate with one another inwalkie-talkie mode. When the user of a terminal according to FIG. 4wishes to access the channel so as to transmit voice signals, heactuates the PTT button 30 with which the terminal is provided. Histerminal, which is not master of the channel at this moment, thenexamines whether the carrier is busy, by listening to the channel forlong enough to pick up at least one time interval of the CS/FS logicalchannel. If the busy state is not detected, the terminal determines thatno other station is master of the channel, and it becomes master itselfso as to transmit the signals of the user. It can also become master ifthe channel is busy with a communication over which the user has a rightof override (which can be determined on the basis of the communication'spriority level indicated in the CS information). The channel is releasedwhen the user has finished speaking. The system thus provides telephonyservices similar to those offered in traditional systems without“trunking”.

[0053] Advantageously, the system also allows communications of callwith off-hook type, between two mobile terminals or further in direct orrepeated mode. One or more of the terminals involved in such acommunication can also be external terminals attached by way of agateway 7. The call is firstly set up in the manner indicatedpreviously. On termination of this setup phase and up to the end of thecommunication (the penultimate station hangs up), there is always one ofthe radio stations involved which is master of the channel.

[0054] While a station is transmitting a voice signal on a carrier, itis master of the corresponding frequency channel. When it stopstransmitting on the TCH logical channel without the communication beingcompleted, it remains master and its generator 16 generates the framestructure according to FIG. 3. The circuits 21 are operated in such away that the master station continues to transmit on the logicalsignaling channel CS, by supplying at least the CS information and thesynchronization pattern as explained previously. This prevents anytrespass on the channel of a terminal not involved in the communicationin progress and having no right of override. Moreover, the masterstation retaining the channel places itself in reception mode in thetime intervals of 40 ms in which it does not transmit the CSinformation.

[0055] The state of the channel coded in the CS information transmittedin the channel retention phase indicates to the slave stations involvedin the communication that they can seize the channel. If the user ofsuch a station actuates his PTT button 30, this station becomes masterof the channel and begins to transmit signals on the CS/FS and TCHlogical channels. To guarantee that the synchronization pattern sent onthe CS/FS logical channel will be detected by the station previouslymaster, the new master station shifts its transmission in such a waythat the first synchronization pattern falls in the third time intervalof 40 ms of the frame previously transmitted in the channel retentionphase (if need be in the second or the fourth). In response to thedetection of signals transmitted on the channel by another station, inparticular the synchronization pattern, the station which was masterbecomes slave and synchronizes itself with the frame structure of thenew master station.

[0056] In the exemplary embodiment illustrated by FIG. 4, the circuits26 for reception processing on the CS/FS logical channel deliver a bit Vwhose activation indicates that the slave station is authorized to seizethe channel. This bit is determined by analyzing the CS informationtransmitted by the master station. It is activated if the state of thechannel indicates that the master station has stopped transmitting onthe TCH logical channel (channel retention phase) and if the slavestation identifies itself as participating in the communication on thebasis of the addresses included in the CS information. It may also beactivated if the terminal notes that it can exercise a right of channeloverride in view of the priority level indicated. This bit V is suppliedto the control unit 15 so as to validate the master status when the useractuates the PTT button 30. Moreover, the unit 15 addresses the bit W tothe frame generator 16, this bit indicating, when a call is in progress,whether or not one of the communicating stations is in the channel usephase (W=1).

[0057] When the station is master (M/S=1) without occupying the TCHlogical channel (W=0), the radio stage 11, the modulator 19 and thedemodulator 13 are operated in a modified manner by the frame generator16 so as to place the station in reception mode in the second, third andfourth time intervals of 40 ms of each frame according to FIG. 3. Thedetection of the synchronization pattern by the unit 12 when M/S=1 andW=0 then causes the station to toggle into the slave state (M/S=0).

[0058] In a variant embodiment, the change of master station of thechannel currently communicating could result from a dialog (handshake)between the station previously master and that which requires to becomeso.

[0059] Call with off-hook functions in a similar manner when a repeater16 is interposed between the master terminal 2 and the slave terminal 5,as illustrated by dashes in FIG. 1. The repeater 6 echoes the framestructure generated by the master terminal 2, with carrier frequencytransposition, to the terminal 5, thereby allowing the latter tosynchronize itself also. Thus, as far as the frequency channel usedbetween the terminal 2 and the repeater 6 is concerned, the terminal 2is the master station transmitting the voice signal and generating theframe structure. On the other frequency channel used between theterminal 5 and the repeater 6, it is the repeater 6 which, relaying theterminal 2, plays the role of master station transmitting voice signalsby using the frame structure with which it has previously synchronizeditself. The repeater 6 can be regarded as comprising twotransmitting/receiving stations back-to-back, one working on the primaryfrequency on which it receives the signals from the master terminal, andthe other on the transposed frequency on which it retransmits them, thedifference with the case of the direct mode described hereinabove beingthat the terminals must switch their frequency for the RS logicalchannel.

1. A method for controlling a channel shared by severaltransmitting/receiving stations, comprising at least one phase of use ofthe channel by a station participating in a communication, in the courseof which said station transmits traffic information on the channel,followed immediately by a phase of retention of the channel by saidstation, in the course of which said station transmits signalinginformation authorizing each other station participating in thecommunication to transmit in turn traffic information on the channel,the phase of retention of the channel by said station continuing, solong as the communication is in progress, until another phase of use ofthe channel by a station participating in the communication.
 2. Themethod as claimed in claim 1, in which, in the phase of use of thechannel, said station furthermore transmits signaling informationrequesting each other station participating in the communication not totransmit traffic information.
 3. The method as claimed in claim 2, inwhich said station operates in the phase of use of the channel accordingto a first frame structure comprising at least time intervals for thetransmission on the channel of the traffic information and timeintervals for the transmission on the channel of the signalinginformation, and in which said station operates in the phase ofretention of the channel according to a second frame structurecomprising at least time intervals for the transmission on the channelof the signaling information and time intervals for the reception ofsignals originating from another station participating in thecommunication and beginning to transmit traffic information on thechannel, the reception of said signals ending the phase of retention ofthe channel by said station, replaced by a phase of use of the channelby said other station.
 4. The method as claimed in claim 3, in which thefirst and second frame structures furthermore comprise time intervalsfor the reception of signals originating from stations not participatingin the communication.
 5. The method as claimed in any one of thepreceding claims, in which said signaling information comprises data foridentifying the stations participating in the communication.
 6. Themethod as claimed in any one of the preceding claims, in which saidsignaling information comprises data indicating a level of priority ofthe communication.
 7. The method as claimed in any one of the precedingclaims, in which said signaling information comprises a reference foridentifying the communication.
 8. A station for transmitting/receivingon a shared channel, comprising: means of use of the channel in mastermode in the course of a communication, which are designed to transmittraffic information on the channel; means of retention of the channel inmaster mode in the course of a communication, which are designed toalternately transmit signaling information on the channel and to listento the channel, said signaling information authorizing each otherstation participating in the communication to transmit in turn trafficinformation on the channel; means of reception in slave mode forreceiving traffic and signaling information transmitted in master modeby another station participating in the communication; and means ofcontrol for activating the means of use of the channel in master mode inresponse to a request for use when signaling information authorizing thetransmission of traffic information has been received by the means ofreception in slave mode, so as to activate the means of retention of thechannel in master mode in response to a deactivation of the means of useof the channel in master mode, and to activate the means of reception inslave mode in response to the reception of signals originating fromanother station participating in the communication by the means ofretention of the channel in master mode listening to the channel.
 9. Thetransmitting/receiving station as claimed in claim 8, in which the meansof use of the channel in master mode are designed to transmit,furthermore, signaling information requesting each other stationparticipating in the communication not to transmit traffic information.10. The transmitting/receiving station as claimed in claim 9, in whichthe means of use of the channel in master mode operate according to afirst frame structure comprising at least time intervals for thetransmission on the channel of the traffic information and timeintervals for the transmission on the channel of the signalinginformation, and in which the means of retention of the channel inmaster mode operate according to a second frame structure comprising atleast time intervals for the transmission on the channel of thesignaling information and time intervals for channel listening.
 11. Thetransmitting/receiving station as claimed in claim 10, in which thefirst and second frame structures furthermore comprise time intervalsfor the reception of signals originating from stations not participatingin the communication.
 12. The transmitting/receiving station as claimedin any one of claims 8 to 11, in which said signaling informationcomprises data for identifying the stations participating in thecommunication.
 13. The transmitting/receiving station as claimed in anyone of claims 8 to 12, in which said signaling information comprisesdata indicating a level of priority of the communication.
 14. Thetransmitting/receiving station as claimed in any one of claims 8 to 13,in which said signaling information comprises a reference foridentifying the communication.